Contact structure for connecting an electrode to a semiconductor device and a method of forming the same

ABSTRACT

A contact structure for connecting a semiconductor device to a wiring electrode includes a semiconductor layer forming a part of the semiconductor device. A first contact layer of reduced resistivity covers a surface of the semiconductor layer. An insulating structure is provided on the first contact layer so as to bury the first contact layer underneath. A penetrating hole is opened through the insulating structure so as to expose a part of the first contact layer. A second contact layer of reduced resistivity is provided on the part of the first contact layer exposed by the penetrating hole. The second contact layer extends from a bottom of the penetrating hole along its side wall. A conductor layer forms the wiring electrode on the second contact layer.

This is a divisional of application Ser. No. 08/115,242, filed on Aug.18, 1993, now U.S. Pat. No. 5,384,485, which is a continuation ofapplication Ser. No. 07/948,622, filed on Sep. 22, 1992, abandoned,which is a continuation of Ser. No. 07/697,748, filed on May 6, 1991,abandoned, which is a continuation of application Ser. No. 07/354,609,filed on May 22, 1989, abandoned.

BACKGROUND OF THE INVENTION

The present invention generally relates to fabrication of semiconductordevices and more particularly to a contact structure for connecting anelectrode to a semiconductor device and a method of forming the same.

Connection of a wiring electrodes to a semiconductor device is usuallyachieved through a contact hole provided on an insulator layer such as asilica glass or phosphosilicate glass (PSG) layer covering a part of thedevice which may be a substrate. In a typical example shown in FIG. 1,an aluminium or aluminium alloy conductor 1 is deposited on a surface ofan insulator layer 2 including area of a contact hole 3 by sputteringand the like so that a part 4a of a substrate 4 exposed by the contacthole 3 at its bottom is covered by the conductor 1. When a layer of theconductor having a substantial thickness is formed continuously from thebottom of the contact hole 3 to the surface of the insulator layer 2along a side wall 3a of the contact hole 3, there is achieved anexcellent electrical contact between the substrate and the wiringelectrode.

As will be easily understood, the key factor for achieving a successfulelectrical contact is to cover the side wall 3a of the contact hole 3uniformly by the conductor having a substantial thickness. However, sucha uniform coverage of the side wall is not achieved easily, as the sidewall 3a of the contact hole generally extends vertically relative to thesubstrate 4 so as to reduce the diameter of the contact hole and hencethe size of the device. When the conductor 1 such as aluminium oraluminium-silicon alloy is deposited by. sputtering as is commonlypracticed, the conductor covering the surface of the insulator layergrows laterally into the contact hole 3 and forms an overhang la. Undersuch circumstances, there is no or little deposition of the conductor atthe bottom part of the contact hole 3, particularly on the side wall 3aeven after a continued effort of deposition. When such a situationoccurs, the layer of conductor 1 covering the side wall 3a becomes verythin as illustrated, and in the most extreme case, there is no conductorlayer covering such portions. The connecting structure having such aprofile is of course unstable and tends to cause disconnection of theelectrical contact.

In order to eliminate the aforementioned problem, it is proposed todeposit a metal 5 such as titanium which forms a silicide when reactedwith silicon, on the insulator layer 2 including the area of the contacthole 3 prior to the deposition of the conductor layer as shown by brokenline in FIG. 2. The device deposited with the metal 5 is then subjectedto heat treatment, whereby a layer of silicide 6 is formed so as tocover the substrate 4 in correspondence to the bottom of the contacthole 3 as a result of reaction between the metal 5 and silicon containedin the substrate 4. Further, silicon is transported upwards along theside wall 3a and there is formed a layer of silicide 6a which extendsalong the side wall 3a of the contact hole. After the heat treatment,the unreacted part of the metal 5 is removed. As the silicide layer 6 isformed by the reaction between the metal and silicon which is suppliedfrom the substrate 4 through the bottom of the contact hole 3, thethickness of the silicide layer 6 is generally largest at the bottom ofthe contact hole 3 and the silicide layer 6a covering the side wall 3aof the contact hole 3 gradually decreases its thickness from the bottomto the top of the contact hole. In other words, the silicide layer 6 hasa concaved inner surface 6b opened upwards. Further, this concavedprofile of the inner surface of the silicide layer is formed withreliability even if there is formed an overhang 5a of the metal layer 5when the metal is initially deposited on the contact hole. This isbecause there occurs a flow of component element constituting the metalin a direction opposite to the direction of the flow of silicon movingupwards from the substrate to the metal layer, irrespective of theinitial profile of the deposited layer 5 of the metal. There is nodifficulty in depositing the conductor layer on such a concaved surface6b of the silicide layer 6 by the conventional procedure such assputtering. By selecting the metal such that the silicide thus formedhas a low resistivity, an excellent contact is achieved between thesemiconductor device and the aluminium-based wiring electrode depositedon the silicide layer.

In such a prior art contact structure, there is formed a thick layer ofsilicide 6c at the bottom of the contact hole as already described. Sucha silicide layer has a bottom surface which is generally not flat buthas many projections or spikes 6d projecting into the substrate 4particularly at a region adjacent to a bottom edge 3b of the side wallof the contact hole. Such a projection 6d is formed in a region of thesubstrate which acts as a source of silicon from which silicon isremoved in exchange with incoming flow of the element of the metal suchas titanium when the silicide layer 6a is formed along the side wall 3aof the contact hole. It should be noted that it is such a region alongthe bottom edge 3b of the side wall 3a that supplies most of siliconwhen the silicide layer 6a is formed along the side wall 3a of thecontact hole 3. In conventional semiconductor devices having arelatively deep p-n junction in the substrate, the existence of such aprojection of the silicide layer does not cause a serious problem.However, in a semiconductor device having a shallow junction representedschematically by a one-dotted line in FIG. 2 as in the case of verylarge scale integrated circuits (VLSI) in which a very large number ofdevices are assembled in a unit area, there is a substantial risk thatthe spike 6d of silicide 6 thus extending into the substrate 4 causesshort-circuit conduction in the shallow junction. In order to avoid sucha problem and at the same time to achieve a reliable electrical contact,one has to prevent the excessive projection of the silicide into thesubstrate.

In a conventional contact structure where the aluminium-based wiringelectrode is directly contacted with silicon substrate through thecontact hole as in FIG. 1, there arises another problem of reactionbetween the silicon substrate and the electrode as a result of diffusionof aluminium and silicon as schematically illustrated in FIG. 1. Whensuch a reaction occurs, a spike of aluminium silicide 4b shown by adotted line in FIG. 1 is formed in the substrate 4 and the p-n junctionin the substrate is shorted. In order to prevent such a reaction, adiffusion barrier layer (not shown) which may be a layer of titaniumnitride (TiN) or titanium tungstenite (TiW) is provided between thesubstrate and the wiring electrode so as to block the transportation ofaluminium or silicon passing therethrough. In such a conventionalcontact structure, there is a problem in that the coverage of thesubstrate at the bottom of the contact hole by the diffusion barrierlayer tends to become insufficient because of the reason similar to thecase of depositing the conductor layer on the substrate through thecontact hole, particularly when the device has a very fine pattern andthe contact hole has a correspondingly large aspect ratio which is aratio of a depth relative to a diameter of the contact hole. Thisproblem is further deteriorated by the limited thickness of thediffusion barrier layer as the thickness of such a diffusion barrierlayer is generally limited below about 3000 Å in order to secure asufficiently low resistance of the contact structure.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful contact structure wherein the problems aforementionedare eliminated.

Another and more specific object of the present invention is to providea contact structure for connecting a semiconductor layer forming a partof a semiconductor device to a wiring electrode wherein a reliablecontact is obtained between the semiconductor layer and the wiringelectrode.

Another object of the present invention is to provide a contactstructure for connecting a semiconductor layer forming a part of asemiconductor device to a wiring electrode via a silicide layer formedat a boundary between the semiconductor layer and the electrode as aresult of reaction between the semiconductor layer and a metal layerdeposited thereon in correspondence to the silicide layer, whereinexcessive projection of the silicide layer into the semiconductor layercausing short circuit conduction in a p-n junction in the semiconductorlayer is eliminated.

Another object of the present invention is to provide a contactstructure for connecting a semiconductor layer forming a part of asemiconductor device to a wiring electrode through a contact holewherein reaction between the semiconductor layer and the wiringelectrode is effectively suppressed by a diffusion barrier layer evenwhen the contact hole has a large aspect ratio.

Another object of the present invention is to provide a contactstructure for connecting a semiconductor layer constituting a part of asemiconductor device to a wiring electrode through a contact holeprovided on an insulator layer covering the semiconductor layer, whereinthere is provided a first contact layer at a boundary between thesemiconductor layer and the insulator layer in correspondence to thecontact hole such that the first contact layer extends laterally alongthe boundary beyond the contact hole. A second contact layer is formedon the first contact layer in correspondence to the contact hole suchthat the second contact layer covers the first contact layer at thebottom of the contact hole and extends upwards from the first contactlayer along a side wall of the contact hole. A diffusion barrier layerprovided on a surface of the second contact layer is so as to besandwiched between the second contact layer and the wiring electrode forpreventing diffusion of component elements of the semiconductor layerand the wiring electrode passing therethrough. According to the contactstructure of the present invention, the first contact layer extendinglaterally along the boundary between the semiconductor layer and theinsulator layer has a substantially flat bottom surface and formation ofa projection or spike projecting into the semiconductor layer to such anextent that a shallow junction formed in the semiconductor layer isshorted by such a projection is avoided. The bottom of the first contactlayer is maintained flat, as silicon is collected from the semiconductorlayer uniformly by the widely extending first contact layer during theformation of the second contact layer. Further, the deposition of thewiring electrode in such a contact structure is facilitated as thesecond contact layer covering the the side wall as well as the bottom ofthe contact hole has a concaved surface opened upwards. Further, as aresult of the existence of the first contact layer extending laterallybeyond the contact hole at the boundary between the insulator layer andthe semiconductor layer, area for electrical connection is increased andthere is achieved an excellent electrical contact between the wiringelectrode deposited on the second contact layer which in turn iscontacted with the first contact layer at the bottom of the contact holeand the semiconductor layer located underneath the first contact layer.As a result of the reduced resistance in the structure, one can reducethe size of the contact hole which in turn enables miniaturization ofthe semiconductor device. Furthermore, the reaction between the wiringelectrode and the semiconductor layer across the first and secondcontact layers is effectively suppressed by the diffusion barrier layerformed on the surface of the second contact layer. In such a structure,the diffusion barrier is easily formed as a thin layer having a uniformthickness by reacting the surface of the second contact layer with asuitable atmospheric gas. Alternatively, the diffusion barrier layer maybe formed by depositing a suitable material on the surface of the secondcontact layer prior to the deposition of the conductor layer. It shouldbe noted that the concaved surface of the second contact layerfacilitates the uniform deposition of such a material.

Another object of the present invention is to provide a method offorming a contact structure for connecting a wiring electrode to asemiconductor layer forming a part of a semiconductor device through acontact hole provided on an insulator layer covering a surface of thesemiconductor layer such that an electrical contact is achieved betweenthe wiring electrode and the semiconductor layer via a first contactlayer at a boundary between the semiconductor layer and the insulatorlayer including area of the contact hole and extending beyond thecontact hole and a second contact layer provided on the first contactlayer in correspondence to the contact hole and extending upwards alonga side wall of the contact hole, wherein the first contact layer isformed at first by reacting a first metal layer deposited incorrespondence to the first contact layer with the semiconductor layerat a relatively low temperature to form a precursor compound. Then thelayer of the precursor compound and a second metal layer deposited incorrespondence to the second contact layer are simultaneously heated ata relatively high temperature so that the precursor compound is changedto a desired compound having a low resistivity. At the same time thesecond contact layer of the desired compound is formed as a result ofreaction between the semiconductor layer and the second metal layeracross the first contact layer. According to the present invention, theheat treatment of the semiconductor device at the high temperature forchanging the precursor compound to the desired compound is applied onlyonce and thus deteriorative effect associated with heating on variousparts of the device is minimized. Further, it is possible to form alayer of diffusion barrier on a surface of the second contact layersimultaneously at the time the first and second contact layers areformed by a reaction with an atmospheric gas. Thus, the step to formsuch a structure is simplified.

Still other objects and further features of the present invention willbecome apparent from the following detailed description when read inconjunction with attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a typical prior art contactstructure and problems associated therewith;

FIG. 2 is a cross sectional view showing a another prior art contactstructure and problems associated therewith;

FIGS. 3(A)-3(E) are diagrams showing steps of forming a MOS transistorhaving a contact structure according to an embodiment of the presentinvention; and

FIGS. 4(A) and 4(B) are diagrams showing steps of forming a MOStransistor having a contact structure according to another embodiment ofthe present invention.

DETAILED DESCRIPTION

FIG. 3(A)-(E) show steps of forming a so called "salicide" orself-aligned silicide contact structure according to an embodiment ofthe present invention for an n-channel type MOS structure. Referring toFIG. 3(A), a known MOS structure is constructed in an area of a p-typesubstrate 11 defined by a field oxide layer 12. The MOS structure has ann⁺ -type source region 14 and an n⁺ -type drain region 15 both formed inthe substrate 11 in correspondence to the area defined by the fieldinsulator layer 12. Doping of the p-type substrate 11 corresponding tothe source and drain regions 14 and 15 may be achieved by well known ionimplantation of arsenic ion (As⁺) or phosphorus ion (P⁺). On thesubstrate 11, there is provided a gate insulator film 13a which may bean oxide film formed as a result of oxidation of the substrate 11, and apolysilicon gate electrode 13 is deposited on the gate insulator film13a. Further, the side wall of the gate electrode 13 is covered byanother insulator layer 13b formed by oxidation of side wall portion ofthe gate electrode 13. In this state, only the source and drain regions14 and 15 as well as the top surface of the gate electrode 13 areexposed.

In a step of FIG. 3(A), titanium is deposited on the aforementioned MOSstructure by sputtering and the like, whereby a layer of titanium 16covering the entire surface of the structure is formed with a thicknessof about 400 Å or more. In the illustrated example, the sputtering oftitanium is made by D.C. magnetron sputtering with a power of 4 kW usingargon gas of about 3 mTorr pressure. The structure thus obtained is thenannealed at a temperature of about 600°-700° C. in an inert atmospheresuch as nitrogen for a short time period such as one minute, whereby aself-aligned structure of silicide shown in FIG. 3(B) is obtained.Such-a heat treatment for very short time period may be achieved byusing a rapid thermal anneal (RTA) technique. Referring to FIG. 3(B),there is formed a layer of silicide 17a in correspondence to the exposedregions 13, 14 and 15 as a result of reaction between titanium in thelayer 16 and silicon in the exposed region underneath. Such a silicidelayer formed by the aforementioned annealing mainly contains titaniummonosilicide TiSi with a small amount of titanium disilicide TiSi₂ andother titanium-based silicide compound such as Ti₅ Si₃. As a whole, thesilicide in the layer 17a obtained in this stage is represented asTiSix.

As is commonly known, layer of silicide formed as such grows along theinsulator layer beyond the exposed region of the substrate. Thus, it canbe seen that the silicide layer 17a not only covers the exposed sourceand drain regions 14 and 15 but extends beyond such regions along thefield insulator layer 12 as well as along the insulator layer 13b at theside wall of the gate electrode 13. When the extension of the layer 17a,particularly those along the side wall 13b of the gate electrode 13 isexcessive, there is a risk that a part of the layer 17a makes contactwith the silicide layer 17a covering the top surface of the gateelectrode 13. By limiting the temperature and duration of the annealingto such a low temperature and extremely short duration, such anexcessive extension of the silicide layer 17a along the side wall 13b ofthe gate electrode 13 is eliminated. After the formation of the silicidelayer 17a, an unreacted part of the titanium layer 16 is removed byetching. The etching may be made by an isotropic etching using asolution of hydrogen peroxide (H₂ O₂) and ammonium hydrate (NH₄ OH) at60° C. with a concentration level which may be chosen as H₂ O₂ : NH₄ OH:H₂ O=1.5:1:4, for example.

Next, a silicon dioxide layer 18 and a PSG layer 19 are depositedsuccessively on the entire structure of FIG. 3(B) so that the silicidelayer 17a is buried under the silicon dioxide layer 18, and a contacthole 20 is opened through the layers 18 and 19 in correspondence to thesource and drain regions 14 and 15 by anisotropic etching such asreactive ion etching (RIE) so as to expose a part of the silicide layer17a covering the regions 13, 14 and 15 as shown in FIG. 3(C), though thecontact hole for the gate electrode 13 is not illustrated for the sakeof clarity of the drawing. It should be noted that the contact hole 20does not expose the entire contact layer 17a but only a part of it. Inother words, the silicide layer 17a extends laterally between beyond thecontact hole 20 along a boundary between the substrate 11 and theinsulator layers 18 and 19. Next, another titanium layer 21 is depositedon the entire structure thus obtained as shown in FIG. 3(C) such thatthe titanium layer 21 covers the exposed contact layer 17a at the bottomof the contact hole 20 as well as the surface of the PSG layer 19including a part defining the side wall of the contact hole 20. Thedeposition of the titanium layer 21 is made similarly to the case of thedeposition of the titanium layer 16. Thus, the thickness of the titaniumlayer 21 is made equal to or larger than about 400 Å.

Next, the entire structure of FIG. 3(C) is annealed at a temperature ofabout 900° C. for several tens of minutes under a nitrogen or ammoniaatmosphere. As a result, another silicide layer 17b is formed along theinner surface of the contact hole 20 as shown in FIG. 3(D) as a resultof reaction between silicon supplied to the titanium layer 21 from thesubstrate 11 through the first silicide layer 17a. Experimentally, it isfound that a same structure is obtained by annealing at a temperature of600°-900° C. Silicon supplied from the substrate 11 through the firstsilicide layer 17a migrates along the side wall of the contact hole, andthe second silicide layer 17b thus formed extends upwards from thebottom along the side wall of the contact hole. In exchange with theflow of silicon, titanium flows from the titanium layer 21 along areversed diffusion path through the first and second silicide layers 17aand 17b to the substrate 11. As the temperature of annealing issubstantially higher than that of the first annealing applied at thetime of formation of the first silicide layer 17a, the degree ofextension of the second silicide layer 17b along the side wall of thecontact hole 20 is much larger than the extension of the first silicidelayer along the field isolation structure 12 or along the insulatorlayer 13b at the side wall of the gate electrode 13. Further, it shouldbe noted that the first silicide layer 17a covering the field isolationlayer 12 or the oxide film 13b of the gate electrode 13 is buried underthe silicon dioxide layer 18 except for those exposed by the contacthole 20, so that there occurs no further growth or extension of thefirst silicide layer 17a beyond the state of FIG. 3(C) even in thesecond annealing performed at a higher temperature. Thus, the risk thatthe first silicide layer 17a covering the source or drain regions 14, 15grows excessively and makes a contact with the silicide layer 17acovering the top surface of the gate electrode 13 is eliminated. By thesecond annealing, the silicide of the first silicide layer 17a mostlycomprised of titanium monosilicide changes to titanium disilicide havinga low resistivity. At the same time as the formation of titaniumdisilicide, the rest of the titanium layer 21 which remains unreactedwith silicon is reacted with nitrogen in the atmosphere and there isformed a layer of titanium nitride 21a acting as a diffusion barrierlayer.

Next, in a step shown in FIG. 3(E), a wiring electrode 22 is depositedon the entire structure of FIG. 3(D) and then patterned together withthe titanium nitride layer 21a according to a desired wiring pattern.Further, the entire structure is protected by another PSG layer 23.

The completed contact structure of FIG. 3(E) has various advantages.First, it has the second silicide layer 17b covering the bottom as wellas the side wall of the contact hole 20 and the first silicide layerextending laterally at the boundary between the substrate 11 and theinsulator layers 18 and 19 beyond the contact hole 20. Thus, there isachieved an excellent electrical contact between the wiring electrode 22and the substrate 11 as a result of increased contact area. Further, thesecond silicide layer 17b has a concaved profile opened upwards in whichthe thickness of the layer is largest in the bottom part of the contacthole 20 and becomes gradually small towards the top along the side wallof the contact hole 20. On a part having such a profile, there is nodifficulty in depositing the wiring electrode 22 by commonly usedtechnique of sputtering and the like. In other words, the deposition ofthe wiring electrode on the second silicide layer 17b can be madewithout causing problem even if the diameter of the contact hole isreduced in association with the miniaturization of the semiconductordevice and the aspect ratio of the contact hole is increasedaccordingly. Further, the contact structure of FIG. 3(E) issubstantially free from spikes or projections projecting from the bottomof the first contact layer 17a into the region 14 or 15 of the substrate11. This is because silicon consumed for the growth of the secondsilicide layer 17b is collected from a wide area of the substrate 11covered by the first silicide layer 17a uniformly. Correspondinglythereto, the first silicide layer 17a grows towards the substrate 11uniformly for a minute distance as a result of supply of titanium fromthe titanium layer 21. Thus, localized growth of the first silicidelayer 17a towards the substrate 11 bringing the formation of the spikeas in the case of prior art structure of FIG. 2 is avoided and theformation of spike or projection which cause short-circuit conductionacross the source or drain region 14, 15 is successfully suppressed. Inthe procedure of FIGS. 3(A)-(E), it should be noted that the annealingof the structure at the high-temperature for substantial period of timeis made Only once, so that deteriorative effect of heating which maychange the distribution profile of impurities in the device isminimized. Associated therewith, the process of formation of the contactstructure is simplified. Further, the diffusion barrier layer 21a isformed simultaneously to the formation of the second contact layer 17b.

FIGS. 4(A) and (B) show an alternative process of forming a self-alignedcontact structure according to a second embodiment of the presentinvention. As the steps corresponding to those of FIGS. 3(A)-(C) arecommon, the illustration and description of such steps will be omitted.Further, those parts constructed identically to these correspondingparts of the preceding drawings are given identical reference numeralsand the description thereof will be omitted.

Referring to FIG. 4(A), the second contact layer 17b is grown on thefirst silicide layer 17a similarly to the case of FIG. 3(D) except thatthe second annealing for the formation of the second silicide layer 17bis made in argon. The temperature and duration of the second annealingis substantially the same as in the case of the first embodiment. Theannealing in argon provides advantage in that there is formed anextensive growth of the second silicide layer 17b along the side wall ofthe contact hole 20. In this embodiment, the titanium nitride layer isnot formed. Thus, after the formation of the second silicide layer 17b,the unreacted part of the titanium layer 21 is removed by etchingsimilarly to the case of the titanium layer 16. Next, a desirablematerial for diffusion barrier such as titanium tungstenite is depositedon the structure of FIG. 4(A) as a diffusion barrier layer 24. After thedeposition of aluminium wiring electrode 22 and patterning together withthe diffusion barrier layer 24, the entire structure is protected by thePSG layer 23 similarly to the case of the first embodiment and acompleted structure is obtained as shown in FIG. 4(B).

In this embodiment, the material for the diffusion barrier layer is notlimited to titanium nitride but any desirable material such as titaniumtungstenite may be used for the diffusion barrier layer. As the secondsilicide layer has the concaved profile opened upwards as alreadydescribed, there is no difficulty in depositing the diffusion barrierlayer by commonly used technique such as sputtering. Further, it is alsopossible to provide the titanium nitride layer as the diffusion layer bychanging the atmosphere from argon to nitrogen when the structure ofFIG. 4(A) is formed. In this case, one can obtain an extensively grownstructure of the second silicide layer 17b along the side wall of thecontact hole 20 in a same processing apparatus by simply changing theatmospheric gas while suppressing the growth of the first silicide layer17a along the side wall 13b of the gate electrode 13 by the insulatorlayers 18 and 19. In this embodiment, too, the formation of projectionor spike into the source or drain region of the substrate 11 as a resultof the formation of the second silicide layer 17b is suppressed as aresult of use of the first silicide layer 17a which spreads the areaused for exchange of titanium and silicon between the the substrate andthe silicide layers.

Further, the compound for the first and second silicide layers is notlimited to titanium disilicide but other compounds may be used as well.The device to which the contact structure of the present invention isapplicable is not limited to MOS transistors as illustrated, but thecontact structure of the present invention is applicable to any otherdevices such as bipolar transistors as well.

Further, the present invention is not limited to these embodiments butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A method of forming a contact structure forconnecting a semiconductor device to a wiring electrode, comprising thesteps of:depositing a first metal on a surface of a semiconductor layerconstituting a part of the semiconductor device to form a first metallayer in contact with the semiconductor layer, said first metal beingchosen such that it reacts with the semiconductor layer when annealed;annealing the first metal layer at a first temperature to form a firstcontact layer as a result of reaction between the semiconductor layerand the first metal; depositing an insulator material on the firstcontact layer to form an insulator layer such that the first contactlayer is buried under the insulator layer; providing a penetrating holethrough the insulator layer so as to expose a part of the first contactlayer; depositing said first metal on the insulator layer incorrespondence to the penetrating hole so as to cover at least said partof the first contact layer exposed by the penetrating hole to form asecond metal layer; annealing at a second temperature higher than thefirst temperature to form a second contact layer as a result of reactiondue to the second temperature between the first metal of the secondmetal layer and the semiconductor layer through the first contact layer;and depositing the wiring electrode on the second metal layer.
 2. Amethod as claimed in claim 1 in which said step of forming the secondcontact layer comprises a step of growing the second contact layer alonga side wall of the penetrating hole.
 3. A method as claimed in claim 1in which said step of annealing to form the second contact layer is inan atmosphere containing nitrogen.
 4. A method as claimed in claim 3 inwhich said step of annealing the structure further comprises a step offorming a layer of nitride compound on a surface of the second contactlayer.
 5. A method as claimed in claim 1 in which said step of annealingto form the second contact layer is in an atmosphere containing argon.6. A method as claimed in claim 5 in which said step of annealing toform the second contact layer further comprises the steps of removingunreacted metal, and depositing a diffusion barrier layer on the secondcontact layer.
 7. A method as claimed in claim 5 further comprising astep of annealing at the second temperature in an atmosphere containingnitrogen after the step of annealing in argon.